When using a measuring tape, wrap it around your wrist just above the wrist bone toward the elbow, and add 1/2" to the size. If you like a slightly tighter fit, add 1/4". This is not for a bangle without an opening. If you use a piece of string or another flexible object, wrap it around the wrist the same way, mark it where it meets the end and then measure it with a ruler.
It's always the inner diameter that's a concern. With 1/2" larger, it will hang nicely below your wrist onto your hand.